Terrain-correction over coastal areas

I am pretty new to SAR processing and I am trying to terrain correct S1 images over coastal regions in Antarctica with SNAP. My images were collected partly over sea partly over land. The DEM I am using is the REMA ( Reference Elevation Model of Antarctica at 8m spatial resolution) that provides altitudes above the WGS84 ellipsoid. On the east coast of the Ross sea REMA shows negative altitude (-40m, -50m are common values) and nodata for the sea.
My doubt is: how SNAP will behave at the land/sea transition? It assumes nodata as 0m asl, thus it will see a sharp transitions in elevation producing artifacts? Or will terrain-correct the SAR image only where DEM has meaningful values leaving uncorrected the nodata part?

I have not find anything on this topic so I hope the question isn’t too much silly.